Heretofore, as a material for power-driven structures such as automobiles, magnesium alloys which are light in weigh have been widely used. To use a magnesium alloy for such structures, its structure-sustaining reliability and safety have to be guaranteed, and for that purpose, high strength magnesium alloys have been proposed.
For example, Patent Document 1 discloses a high strength magnesium alloy comprising (a) 4 to 15% by mass of Gd or Dy, and (b) 0.8 to 5% by mass of at least one element selected from the group consisting of Ca, Y and Lanthanoids [provided that the component (a) is excluded], and further, if desired, 2% by mass or less of at least one element selected from the group consisting of Zr and Mn, and the balanced amount of Mg. This high strength magnesium alloy is produced by subjecting materials of the above composition for forging to homogenizing treatment at 430 to 570° C. for 2 to 7 hours, warm forging the materials for forging at a temperature of the materials for forging of 380 to 570° C. and at a mold temperature of 250 to 400° C. which is lower than the temperature of the materials for forging, and age-hardening the obtained warm forged product at 180 to 290° C. for 2 to 400 hours.
Further, Patent Document 2 discloses a high strength magnesium alloy wherein the average composition of the entire alloy is represented by the compositional formula Mg100-a-bLnaZnb (wherein Ln is at least one rare earth element selected from Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and a misch metal, 0.5≦a≦5, 0.2≦b≦0.4 and 1.5≦a+b≦7), and the average crystal grain diameter of the mother phase is 5 μm or less. In this high strength magnesium alloy, in part of crystal grains of the mother phase, a concentration modulation such that the concentration is changed in the crystal grains without precipitation of a new compound is present, and the total concentration of the rare earth element (Ln) is increased by 1 to 6 atomic % and/or the concentration of Zn is increased by 1 to 6 atomic % as compared with the average composition of the entire alloy. This high strength magnesium alloy is produced by rapidly solidifying a molten magnesium alloy having the above composition at a cooling rate of 100 K/s or higher, preparing a powdery alloy having an average grain diameter of about 30 μm by means of a grinder such as a rotor mill, filling an extrusion container with the powdery alloy, and carrying out extrusion with an extrusion ratio (by sectional area ratio) of 3 to 20 with heating. Further, this high strength magnesium alloy has a tensile elongation of 3 to 4%.
Further, Patent Document 3 discloses a high strength magnesium alloy produced by subjecting a magnesium alloy such as a Mg—Zn—Zr system, e.g. ZK60, a Mg—Al—Zn system, e.g. AZ61, or a Mg—Mn system to liquidizing treatment, applying a pre-strain of at least 0.4 in a temperature region of 250 to 400° C. in a first forging step, then carrying out aging, and then carrying out second forging at a predetermined temperature not higher than the temperature at the above forging step, so that the alloy has a fine crystal grain structure with an average crystal grain diameter of 10 μm or less. According to the invention disclosed in the publication, the component segregation is eliminated by carrying out liquidizing treatment so that the magnesium compound which has been unevenly precipitated in the material is sufficiently solid-solubilized in the structure. Then, a predetermined pre-strain is applied to the material in the forging step, and in the subsequent aging treatment, spherical fine grains of the magnesium compound having a small aspect ratio are precipitated, to uniformalize the structure. Then, by the precipitated fine grains, in the forging step, the crystal grain growth in the super-heating procedure to a temperature at which the material is forged is inhibited, whereby stable fine crystal grain structure is formed by the crystal grain refinement effect by the forging.
Whereas, Non-Patent Document 1 discloses a Mg-0.9% by mass Ca (corresponding to 0.55 atomic %) cast material, and the effect of addition of a small amount of Ca to Mg is discussed. To this magnesium alloy, not any other heat treatment is applied. This magnesium alloy has a yield strength at room temperature of about 100 MPa and a tensile elongation of several %. The strengthening mechanism is by precipitation strengthening due to the lamellar phase of Mg2Ca, but the ductility is very low due to the presence of precipitates having a high volume fraction.
Further, Non-Patent Document 2 discloses Mg—Y binary cast alloys having Y concentrations of 5 and 8% by mass (corresponding to 1.4 and 2.2 atomic %, respectively), and reports the yield strengths of the cast material and a T6 aging-treated material. The yield strengths of the alloy containing 8% by mass of Y are about 130 MPa and about 240 MPa, respectively as the cast material and the T6 aging-treated material, and the ductility is not disclosed. High strengthening of this alloy is also due to precipitates.
Patent Document 1: Japanese Patent Application Laid-Open No. 9-263871
Patent Document 2: Japanese Patent Application Laid-Open No. 2004-99941
Patent Document 3: Japanese Patent Application Laid-Open No. 2003-277899
Non-Patent Document 1: Materials Transaction Vol. 43, No. 10 (2002), p. 2643 to 2646 (Yasumasa Chino et al.)
Non-Patent Document 2: Materials Transaction Vol. 42, No. 7 (2001), p. 1332 to 1338 (Si-Young Chang et al.)